Position determination systems are rapidly becoming more prevalent, as position location capabilities may now be found in an increasing number of new mobile handsets on the market. Position location technologies typically utilize wireless positioning signals transmitted from known locations. One widely used system of position determination is the Global Positioning System (“GPS”).
In GPS systems, the positioning signals are concurrently transmitted from a number of satellite vehicles (“SVs”) at known times, with each positioning signal transmitted at predefined carrier frequencies. On the ground, a GPS receiver attempts to acquire positioning signals from the SVs within its view. The times of arrival of the positioning signals, along with the location of the SVs and the times the signals were transmitted from each SV, are used to triangulate the position of the GPS receiver.
For civilian use, the SVs each transmit the positioning signals in the same carrier frequency (1575.42 MHz), and a C/A code modulates the carrier at 1.023 MHz, thereby spreading the signal over approximately a 1 MHz bandwidth. These positioning signals from the SVs each have a repetition period of 1023 chips, for a code period of 1 ms (i.e., 1,023 chips/1.023 MHz). Each SV has a different code, and because the receiver knows such codes, it can acquire a desired positioning signal from a number of received positioning signals. However, a side effect of the GPS C/A code design is that integrating across several 1 ms code periods generally does not significantly improve the processing gain for cross-correlation. This is because the same C/A code sequence is repeated every 1 ms; therefore, while the desired signal is coherently integrated, so is the interfering signal. Perfect coherent integration occurs when the target SV and the interfering SV have the same Doppler, or alternatively when the Doppler difference between them is close to an integer multiple of 1 kHz. This perfect coherent integration is generally not an issue of concern if the positioning signals are received with approximately the same power, as there is usually sufficient spreading gain.
The GPS signal structure is formatted to ensure that the “multiple access interference,” i.e. the noise floor increase due to all satellites sharing essentially the same frequency range, stays below certain levels. This, however, assumes that all SV signals are received at approximately the same power level. But in many cases, the SV signals are received at various power levels. This could occur, for example, when there is a strong signal received through a window, which may interfere with the reception of much weaker signals attenuated by walls. Often the reception of the weaker signals is needed in order to achieve a fully determined position location. Therefore, when there is a sufficient power imbalance, and relative Doppler offset aligns (or when the Doppler offset is close to an integer multiple of 1 kHz), the interfering signal may prevent acquisition of the desired lower power SV positioning signals.
While differing navigation bit sequences between the desired lower power SV signal and the interfering higher power SV signal may give moderate reduction in cross-correlation, the problem remains in many instances. Given the number of visible SVs in the GPS constellation and the resulting number of SV pairs, such undesirable cross-correlation scenarios are likely to occur with regularity.
One suggested solution is to detect suspected cross-correlation cases based on power imbalance and relative Doppler offset, and exclude the suspected measurements from the navigation solution. This method results in improved reliability. However, because it is usually necessary to acquire signals from three or four SVs to determine position location, excluding measurement of the weaker signals may prevent location determination. Thus, it would be desirable to have alternative techniques for suppressing the cross-correlation effects of positioning signals in certain instances.